You could run a small snippet to pre-cache bitmaps using RPG::Cache when the program is first launched. It would add a few seconds to the initial startup time of the game, but would allow for a little smoother loading of maps, etc. Depending on the size of the game and number of bitmaps, this may not be a viable option due to RAM usage, but for most RM games, that really wouldn't be an issue. At the very least you could just load some of the larger files, such as tilesets.
Like I said, it may not be a dramatic change, but would help.